In conjunction with firing a propellant gas-driven projectile from a barrel that is closed at the rear in the direction of firing, a certain initial propellant gas pressure is first required behind the projectile in order to begin to accelerate it along the barrel. Given that the part of the volume of the barrel situated behind the projectile increases successively as the projectile moves along the barrel, quantities of propellant gas which increase to a corresponding degree will be required successively during firing in order continuously to increase the velocity of the projectile for as long as it remains in the barrel. Accordingly, the ideal propellant charge would, as it burns, successively provide increasingly large quantities of propellant gas per unit of time, although in conjunction with this it must not at any time give a propellant gas pressure inside the barrel in question which exceeds the maximum permissible barrel pressure Pmax applicable to the barrel and to parts of the mechanism associated therewith. The entire propellant charge should also be fully expended when the projectile leaves the barrel, as the trajectory of the projectile can otherwise be disrupted by the exiting propellant gases, at the same time as the propellant charge cannot be fully utilized for the intended purpose.
A propellant which, as it burns under constant pressure, gives off a quantity of propellant gas per unit of time, which increases successively with the combustion time, is said to be progressive. The propellant may, for example, have acquired its progressive characteristics as a consequence of a specific geometrical form which presents a combustion area which increases the longer combustion of the same continues, although it may also have acquired its progressive characteristics as a consequence of a chemical or physical surface treatment of parts of the free surfaces of the individual grains of propellant or pieces of propellant contained in the propellant that are accessible for ignition. Propellant charges with at least limited progressive characteristics can thus be produced from granular propellant simply by the choice of an appropriate geometrical form for the grains of propellant contained in the charge.
Granular, single-perforated or multi-perforated propellants provided with through combustion channels or perforations in the longitudinal direction of the propellant grains are ignited and burn both internally in their respective perforations or combustion channels, and from the outside of the propellant grains. This means that there will be a successive increase in the inner combustion areas of the channels, and consequently in the generation of propellant gas therefrom, although at the same time the outer combustion areas of the propellant grains will be reduced as the propellant is also burnt from the outsides of the propellant grains, which gives a reduction in the generation of propellant gas from these surfaces. In order for a granular perforated propellant of this kind to be truly geometrically progressive, there is accordingly a requirement for the successive increase in the propellant channels' own combustion areas actually to exceed the simultaneous successive reduction in the outer combustion areas of the propellant grains. An externally untreated single-perforation propellant with the outer form of a true cylinder normally burns at a constant rate for this reason, whereas a 19-perforation propellant with the external form of a round bar, and similarly untreated, will normally burn progressively.
Also previously disclosed for a long time is the ability to increase the progressivity of a granular multi-perforation propellant, and also to make a single-perforation propellant progressive, by the inhibition or chemical surface treatment of the outer surfaces of the propellant grains. In conjunction with inhibition, the outer combustion areas of the propellant grains are coated with a less readily-combustible substance which delays the propagation of the ignition of the propellant along its surfaces, and in the case of surface treatment, the same surfaces are treated with an appropriate chemical substance which causes the propellant to burn more slowly along these surfaces and for a certain distance into the propellant. In accordance with a third variant, the propellant can be made progressive by coating its outer surfaces with a layer of a propellant which requires to be burnt away first before propagation of the ignition of the outer surfaces of the grains or pieces of the actual propellant charge can take place.
For a number of years, intensive work has been carried out into increasing the performance of older artillery pieces by providing them with more up-to-date ammunition. An initial limiting factor has been the stipulation that the maximum permissible barrel pressure Pmax must never be exceeded. A second previously limiting factor has been that increased performance tends to require an increased charge weight in a charge space that is already fully utilized as a rule in the case of the originally existing charges of loose granular perforated propellant. A third limitation is also that a high charge density requires a progressivity which increases in parallel.
In the case of loose granular material, however, the combined empty volume between the grains is proportionately large. One possibility would thus be to increase the density of the charge. The greatest quantity of propellant, and thus the greatest charge density and the greatest charge weight, that can be accommodated in a fixed volume is a solid body with a geometry that is adapted entirely in accordance with the available volume. However, an entirely solid body of propellant does not offer a general solution to the problem of increasing the performance of existing artillery pieces. The solid body of propellant will burn for too long, in fact, and will produce a propellant gas pressure that is too low to be utilized effectively to propel projectiles.
However, from a theoretical point of view, it is possible to conceive of producing a highly perforated block propellant which burns in a similar fashion to a larger quantity of granular multi-perforated propellant. This is not as simple in practice, however. The theoretically conceived highly perforated block propellant must accordingly be provided in its entirety with a very large number of combustion channels running in parallel, all of which are located at a distance from all adjacent combustion channels equivalent to twice the distance for which the propellant has time to burn during the period available until immediately before the time at which the projectile is intended to have exited from the barrel from which it has been fired. The distance between two combustion channels in a specific propellant is referred to as its e-dimension, and the e-dimension for the propellant that is contained in a specific charge should correspond to the distance for which the propellant has time to burn, during the firing of a specific projectile from the time of ignition until the tire at which the projectile exits from the barrel, with complete combustion during the dynamic pressure sequence in the particular artillery piece for which the propellant is intended. In order for a highly perforated propellant to be capable of being utilized optimally, it is necessary, therefore, for two adjacent perforations or combustion channels to be separated from one another by the distance of the e-dimension which is relevant in each individual case. In order to ensure the best possible firing result, the combustion time of the propellant in barrel weapons must be neither too short, as the maximum barrel pressure will then be exceeded, nor too long, as unburned propellant will then be expelled from the barrel without contributing to the acceleration of the projectile.
In the case of both the well-inhibited, granular perforated propellant and the highly perforated block propellant, the propellant ignites in all of its combustion channels, and burns radially outwards from each respective combustion channel towards the others. Thus, if the right e-dimension has been selected, the combustion surfaces from the different combustion channels will meet immediately before the passage of the projectile through the muzzle. In order to ensure that the combustion of the propellant from the outer parts of the propellant grains does not interfere with the geometrical progressivity, all of the outer propellant surfaces must ideally be inhibited, surface treated or surface coated for this purpose, including the propellant surfaces alongside the perforations.
Presented in our Swedish patent application SE0303301-6 referred to in the introduction is a new type of propellant charge for barrel weapons constructed from one, two or more propellant tubes perforated radially at selected e-dimension distances and arranged inside one another and/or after one another, which tubes burn with a certain overlap that has been achieved by the one or more tubes that are to come later in the combustion chain having been inhibited, surface treated or surface coated along all their outer surfaces in order to delay the propagation of ignition along these surfaces.
The starting material for this charge is thus highly perforated propellant tubes which have been inhibited, surface treated or surface coated, as required, in order subsequently to be arranged concentrically inside one another and/or after one another.
One difficulty encountered in the production of this type of charge is how to make the radially perforated propellant tubes. Thus, in order to be capable of being used and giving the desired result, the e-dimension at the perforations in the propellant tubes must normally lie between 0.5 mm and 10 mm, but preferably between 1 mm and 4 mm, depending on the barrel system. In order to give the desired result in the charges in question, the propellant tubes must also be perforated radially. Furthermore, the requirements for the perforation to be executed in a uniform fashion must be set very high.